class Solution:
def oddEvenList(self, head: ListNode) -> ListNode:
if not head or not head.next:
return head
i, j = head, head.next #奇链和偶链的工作指针
head2 = head.next #记录偶链头防丢失（奇链头为head）
while j.next and j.next.next: #奇链走两步，偶链也走两步
i.next = i.next.next
i = i.next
j.next = j.next.next
j = j.next
if j.next: #长度为奇，奇链多出一个
i.next = i.next.next
i = i.next
j.next = None
i.next = head2 #奇链接上偶链
return head
